Low Temperature Polycrystalline Silicon technology has been widely used in fabricating thin film transistor liquid crystal display (TFT-LCD) devices. Typically, a thin film transistor includes a channel region formed by a polysilicon layer. For forming a polysilicon layer during the TFT-LCD process, an amorphous silicon layer is first formed on a substrate and then an excimer laser annealing process is performed to enable amorphous silicon layer to be transformed into a polysilicon layer.
FIGS. 1A and 1B are cross-sectional diagrams illustrating the TFT-LCD process of manufacturing a display device in accordance with prior art. Referring to FIG. 1A, a substrate 120 is provided, which includes a driving circuit region 100 and a pixel region 110. Next, a nitride layer 130 and an oxide layer 132 are formed on the substrate. Next, a polysilicon island 140 corresponding to a NMOS device and a polysilicon island 142 corresponding to a PMOS device are formed in the driving circuit region 100, and a polysilicon island 144 is formed in pixel region 110. Typically, the method of forming the polysilicon islands 140, 142, and 144 may include, for example, the following steps: forming an amorphous silicon layer; performing an excimer laser annealing process to recrystallize the amorphous silicon layer into a polysilicon layer; and performing the conventional photolithographic and etching process on the polysilicon layer to form the polysilicon islands 140, 142, and 144.
Next, a NMOS channel-doping step is performed to adjust a threshold voltage of the formed NMOS device. Referring to FIG. 1B, a patterned mask 150 is formed on the substrate 120 to cover the polysilicon island 142 by using a photomask (not shown). Next, an ion implantation step 160 is performed to implant ions (such as Boron ions) into the polysilicon islands 140 and 144 by using the patterned mask 150 as a mask. Generally, the ion implantation process performed in the NMOS channel-doping step 160 has an energy of 10 KeV to 15 KeV.
It can be known from the above description with reference to the accompanying drawings that, in the conventional TFT-LCD process, one photomask is required to form a mask layer to cover the PMOS device before performing the NMOS channel-doping step. Therefore, both the process time and cost are increased.
Therefore, it is desired to have a method of forming a display device capable of reducing the required number of the photomasks without affecting yield.